This invention relates to a method of and apparatus for monitoring partial discharge activity in an insulating medium.
Partial discharges occur in insulating media such as the insulation of electrical cables, where an entrapped void of another material (usually a gas) is present in the medium. The presence of the void causes a disturbance in the electric field distribution in the insulating material, with the effect that electrical breakdown of the void can occur independently of the breakdown of the insulating material as a whole. The resulting local flashover or discharge is referred to as a partial discharge. This effect is largely capacitive in nature, and partial discharges typically occur in alternating current electrical distribution systems where the applied voltages are sufficiently high to cause a problem.
Prolonged and/or excessive partial discharge activity in an insulating medium will tend to cause eventual degradation and failure of the insulation. Thus, it is desirable to monitor partial discharge activity to provide an early warning of possible insulation failure. Conventional equipment for measuring partial discharges is illustrated in FIG. 1 of the drawings. The equipment is connected to an AC mains source and comprises a variac 1 which controls the input voltage to a step-up power transformer 2. The secondary winding of the step-up transformer 2 is connected in series with a sample 3 to be measured, a partial discharge-free inductance 4 and a measuring impedance 5. A standard capacitor 6 is connected in parallel with the sample 3 and the measuring impedance 5. The capacitor 6 provides a return path for current at pulse frequencies, while the inductance blocks high frequencies.
The measuring impedance 5 is typically a medium frequency RLC resonant circuit which has the effect of extending the duration of the partial discharge waveform, making it more easily measurable. An oscilloscope 7 is fed with a signal corresponding to the low frequency exciting waveform as well as a phase-shifted version thereof, to produce a Lissajou figure on its screen, with the voltage measured across the measuring impedance 5 superimposed on it. This allows a rough impression to be gained of the timing of the partial discharge relative to the exciting waveform. The height (voltage) of the partial discharge pulses corresponds to the charge transferred, allowing the intensity of the pulses to be calculated.
In practice, accurate measurement of such partial discharges is difficult, due to the very small magnitude of the partial discharges (typically several pico coulombs). This creates problems in terms of the sensitivity and signal to noise capabilities of measuring equipment. Furthermore, the timing of the partial discharges tends to be somewhat random. By this is meant that the timing variation (timing jitter) of successive partial discharges of respective voids is likely to be several orders of magnitude greater than the length of each partial discharge event. This makes digital auto-correlation techniques, in particular, very difficult. A further problem arises from the extremely high frequencies of the partial discharge pulses. Since it is usually desirable to digitise the partial discharge waveforms for analysis, the combination of timing jitter and the presence of very high frequency components in the partial discharge pulses requires the use of extremely high speed digitisation equipment with attendant problems of expense and excessive memory requirements. The greater the timing jitter of the partial discharge pulses, the larger must be the capture "window" of the digitisation apparatus, which requires a proportional increase in memory.